Effect of ultrasound on the properties of fruits and vegetables

Fruits and vegetables undergo a series of changes after harvest due their environment, nutrient supply and injury during the harvesting process. Metabolism is modified but continues to function in live plant tissues. The intention of food preservation methods is to extend the food’s shelf life through the inhibition of deleterious enzymatic reactions and microbial growth and the preservation of nutritional and sensory aspects. Thus, it is important to evaluate the consequences of new methods on the quality of fruits and vegetables.

1.1. Inactivation of enzymes

The evaluation of enzymes and intermediate compounds of the main metabolic pathways or secondary routes can be excellent indicators of product quality, stress condition or physiological disorder. The most important enzymes in fruits and vegetables are involved in taste and odour, such as lipoxygenase, lipases and proteases; texture is related to pectinases, β-glucosidases, cellulases, hemicellulases and peroxidase; colour is related to anthocyanases, peroxidase, lipoxygenase, chlorophyillase and polyphenoloxidase; and nutritional value is related to ascorbic acid oxidase, thiaminase and lipoxygenase (Lamikanra, 2002).

Studies evaluating the effects of ultrasound on the enzymes present in fruits and vegetables are scarce. Most studies are conducted using liquid mediums or foods, such as fruit juices and milk. Nevertheless, this article will discuss the effect of ultrasound on important enzymes that are also present in fruits and vegetables.

The mechanism of enzyme inactivation by ultrasound is associated with cavitation through mechanical and chemical effects (Mañas et al., 2006, Raviyan et al., 2005,Rawson et al., 2011b and Vercet et al., 2001). When the bubbles collapse, high temperatures and shockwave pressures are generated. In addition to these effects, microstreaming is associated with high shear forces. Under these intense conditions, ultrasound could induce the disruption of hydrogen bonding and vander Waals interactions in polypeptide chains, leading to alterations of the secondary and tertiary structures of the protein. The biological activities of enzymes are lost by these modifications. The hot spots also lead to water molecule cleavage, generating high-energy intermediates, such as hydroxyl and hydrogen-free radicals. The free radicals formed might react with some amino acid residues that participate in enzyme stability, substrate binding or catalytic function that consequently modify biological activity (Mawson et al., 2011, Raviyan et al., 2005 and Rawson et al., 2011b).

Some enzymes in food need to be inactivated to ensure them stabilization. The ultrasound inactivation of enzymes depends on properties, such as frequency and power, and factors, such as enzyme type, concentration, medium pH, and temperature (Mawson et al., 2011, Raviyan et al., 2005 and Tarun et al., 2006). Mawson et al. (2011)proposed that hydroxyl radicals produced by cavitation events are less effective on enzymes immobilized on the surfaces or within cells, such as those of vegetable and fruit tissues. However, there is no evidence of the genotoxic potential of ultrasound, as it is unclear whether extracellular or intracellular cavitation occurs.

1.2. Effect on food components

There has been increasing consumer interest in functional food with basic nutrient functions and properties that can promote health and prevent diseases. Fruits and vegetables have these properties. The content of these food compounds is strongly associated with their processing, manipulation and storage. Some studies have shown that these operations have significant effects on the content of bioactive compounds (Plaza et al., 2011, Rawson et al., 2011b, Rawson et al., 2011a, Ruíz-Cruz et al., 2007b, Soria and Villamiel, 2010 and Vandekinderen et al., 2009). Fresh-cut produce deteriorates faster than intact produce, and this deterioration affects not only microbiological quality but also nutritional and sensory quality. Therefore, we know that bioactive compounds decrease as a consequence of deterioration.

It is known that to control microbial contamination in fruits and vegetables, it is necessary to use chemical or physical agents, such as sanitizers. However, it is important that these processing steps and postharvest treatments do not cause a significant reduction in the produce’s nutrient content (Vandekinderen et al., 2008 and Vandekinderen et al., 2009). These characteristics are relevant to a complete evaluation of the effectiveness of the decontamination step.

The food industry is constantly searching for processing technologies that allow the microbiological control of their products without changes in the nutritional and sensory characteristics of fresh product. Studies assessing the effect on nutritional content using ultrasound are typically performed with fruit juices. However, it is important to evaluate these parameters for products subject to new processing technologies to allow a more accurate evaluation of the potential for application in food industry.

Alexandre, Brandão, and Silva (2012) treated strawberries with non-thermal technologies, such as ultrasound (35 kHz), and observed higher anthocyanin contents than in samples washed with chemical solutions when stored at room temperature for 6 days. Tiwari, O’Donnell, Patras, Brunton, and Cullen (2009) studied the stability of anthocyanins in sonicated (20 kHz) strawberry juice during storage and observed that this compound presented high levels of retention. Tiwari et al. (2010) observed the significant retention of anthocyanins in grape juices after treatment with ultrasound (at a constant frequency of 20 kHz and pulse durations of 5 s on and 5 s off): 97.5% AC (cyanidin), 48.2% MA (malvanidina) and 80.9% DA (delphinidin). Tiwari et al. (2009)commented that the degradation of the anthocyanins might be related to oxidation reactions promoted by the interaction of free radicals formed during sonication.

The quality of fruits and vegetables is based on several properties: texture, colour, flavour, and nutritional and functional characteristics. Biochemical transformations during the post-harvest development are primarily responsible for changes in the nutritional and sensory attributes of fruits and vegetables.

In peppers, ultrasound (47 kHz) might injure the cell wall structure and induce changes in the texture (Gabaldón-Leyva et al., 2007). However, Cao et al. (2010) observed that ultrasound treatment (40 kHz) inhibited the decrease of firmness in strawberries. Alexandre et al. (2012) treated strawberries with different sanitizers and observed that samples treated with ultrasound (35 kHz) had 16% more firmness retention than the water-washed samples. These different observations should be due to different treatment conditions as frequency, time, amplitude.

Aday, Temizkan, Büyükcan, and Caner (2013) verified that all ultrasound treatments were effective to reduce mould growth and 30 W and 60 W treatments maintained better textural properties compared with 90 W. As a result, it was concluded that power levels between 30 W and 60 W had improved quality and can be used to extend shelf life of strawberry.